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Java Interview Questions

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Q1
What are the main features of Java?

Java is a high-level, object-oriented programming language designed to be platform-independent via the JVM (Java Virtual Machine).

Its core features include strong memory management, robust security, multithreading support, and a rich API.

Java follows 'write once, run anywhere' philosophy by compiling code into bytecode executed by the JVM.

It's widely used in enterprise applications, Android development, and backend systems.

Q2
What is the difference between JDK, JRE, and JVM?

JVM (Java Virtual Machine): Executes Java bytecode.

JRE (Java Runtime Environment): Includes the JVM and runtime libraries needed to run Java applications.

JDK (Java Development Kit): Contains JRE + tools like javac (compiler), java (interpreter), and jar (packaging tool).

Developers use JDK, end users typically only need JRE, and JVM is the engine behind execution.

Q3
What is the difference between abstract class and interface in Java?

An abstract class can have both abstract and concrete methods, instance variables, and constructors.

An interface contains only abstract methods (before Java 8) and constants.

From Java 8+, interfaces can have default and static methods.

A class can extend only one abstract class but implement multiple interfaces.

Abstract classes are used for shared implementation, while interfaces define contracts.

Q4
What is polymorphism in Java and how is it implemented?
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Polymorphism means 'many forms' and allows objects of different classes to be treated as objects of a common superclass.

In Java, it is achieved through method overriding (runtime polymorphism) and method overloading (compile-time polymorphism).

Method overriding allows subclasses to provide specific implementations of inherited methods.

Polymorphism enables flexible and reusable code through abstraction and inheritance.

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